An integrated amplifier circuit such as an opamp is usually constructed from a small chip of semiconductor material upon which an array of active/passive components have been constructed and connected together to form a functioning circuit. An integrated amplifier circuit is generally encapsulated in a plastic housing (chip) with signal, power supply, and control pins accessible for connection to external electronic circuitry. Typically, input signals transmitted to the integrated amplifier circuit via selected input pins are processed by active and passive components in different stages, e.g., input and turn-around, and the processed signals are then applied to selected output pins using an output stage.
The enormous growth of high-speed communication and high data rate image processing applications, requiring high-speed, low power and small size, has created a demand for miniaturized high-speed amplifiers that can operate at low voltages. To maximize the dynamic range at low supply voltages for this use, it is desirable that the output voltage range for this type of amplifier should be as large as possible. Preferably, the output voltage range of the amplifier would extend from one rail to the other rail of the power supply.
Class-AB circuitry is used in amplifiers that employ both bipolar and MOS components. A Class AB circuit can deliver to and pull from a load a current that is larger than the DC quiescent current flowing into the circuit. For example, the drive current in a Class AB circuit may be 100 milliamps and the quiescent current could be 1 milliamp. Class AB circuitry is preferred in output stage of a low-power high speed amplifier because it improves power efficiency by maximizing the output drive current with a relatively low quiescent current. Also, class-AB circuitry enables the output stage to exhibit good linearity over the entire output voltage range.
However, class-AB circuitry can be prone to cross-over distortion caused by the non-linear operation of transistors in a push-pull amplifier. For example, when changing (crossing over) the active operation of one transistor (turn off) to another transistor (turn on) in an output stage, distortion of the output signal can be caused by a less than ideal (non-linear) operation of a transistor. In FIG. 1, a graph illustrates the effects of crossover distortion in an output signal.
To minimize power consumption when a low-power high-speed amplifier is operated at higher supply voltages, the quiescent current of the amplifier's output stage should remain constant when the supply voltage increases from a minimum to a maximum predetermined voltage. However, stabilization of this quiescent current over a range of supply voltages for an amplifier circuit typically requires some type of compensation for the Early effect (second order effect) which can cause increased quiescent current at elevated supply voltages. Typically, as the physical size of the base width of a transistor is decreased so will the Early voltage, which causes the collector current to increase as collector emitter voltage increases.
FIG. 2 illustrates a graph of the effect of the Early voltage on the operation of a transistor. The x-axis represents values for a collector emitter voltage and the y-axis represents a collector current. Two different Early voltages V.sub.A1 and V.sub.A2 are indicated on the x-axis at 100 volts and 20 volts, respectively. When the Early voltage has a value of 100 volts (V.sub.A1), the collector current does not significantly increase as the collector emitter voltage increases. Alternatively, when the base width is significantly decreased and the Early voltage is decreased to 20 volts (V.sub.A2), the collector current increases steeply as the collector emitter voltage increases.
Compensation for the Early effect has often been provided by physically matching sizes of NPN and PNP bipolar transistors in an integrated circuit. However, matching the sizes of bipolar transistors of different polarities is a process dependent technique that can be relatively imprecise in a highly miniaturized integrated circuit.